555 Timer Astable Calculator
Get the frequency, period and duty cycle of an NE555 astable oscillator.
Waveforms
Formulas
tH = 0.693·(R1+R2)·C, tL = 0.693·R2·C, f = 1.44 / ((R1 + 2R2)·C). In the basic circuit the duty cycle is always above 50%. For 50% or less, add a diode across R2.
Frequently asked questions
Where do 0.693 and 1.44 in the formulas come from?
0.693 is ln 2. The capacitor charges from 1/3 to 2/3 of the supply (or discharges back) in 0.693·R·C. And 1.44 is simply 1/0.693.
Can I get a 50% duty cycle?
Not with the basic circuit, because charging goes through both R1 and R2, so the duty cycle is always above 50%. Put a diode across R2 so the capacitor charges through R1 only and discharges through R2 only, then make R1 equal to R2 for close to 50%.